1. Field of the Invention
The present invention relates to a process for producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation. More particularly, the present invention is concerned with a process for producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation at a temperature in the range of from 380 to 500.degree. C. (ammoxidation reaction temperature) in a fluidized-bed reactor containing a catalyst composition preheated to a temperature of not lower than 300.degree. C. and lower than the ammoxidation reaction temperature, the catalyst composition comprising a carrier having supported thereon an oxide catalyst comprising a compound oxide of molybdenum, vanadium, niobium and at least one element selected from the group consisting of tellurium and antimony, wherein the ammoxidation of propane or isobutane at the ammoxidation reaction temperature is preceded by a specific temperature elevation operation in which the temperature of the preheated catalyst composition in the fluidized-bed reactor is elevated, while supplying into the fluidized-bed reactor a molecular oxygen-containing gas together with a combustible gas capable of combustion by reaction with the molecular oxygen in the presence of the catalyst composition, until the temperature of the catalyst composition reaches the ammoxidation reaction temperature. By virtue of the above-mentioned specific temperature elevation operation using both a molecular oxygen-containing gas and a combustible gas when the temperature of the catalyst composition is 300.degree. C. or higher, the process of the present invention is advantageous in that the temperature elevation of a catalyst comprising a compound oxide of molybdenum, vanadium, niobium and at least one element selected from tellurium and antimony can be performed without suffering a deterioration of the catalytic activity during the temperature elevation of the catalyst, thereby allowing the catalyst to fully exhibit its inherent performance.
2. Prior art
It has been well known to produce an unsaturated nitrile, such as acrylonitrile or methacrylonitrile, by ammoxidation of an olefin, such as propylene or isobutene, namely, by reacting an olefin with ammonia and molecular oxygen.
On the other hand, from the viewpoint of reducing the cost for raw materials, attention has been attracted to a process for producing an unsaturated nitrile (such as acrylonitrile or methacrylonitrile) from an alkane (such as propane and isobutane), which is available at low cost as compared to an alkene, by ammoxidation in the presence of a catalyst, i.e., by reacting an alkane with ammonia and molecular oxygen in the presence of a catalyst. Further, a number of proposals have also been made with respect to catalysts for use in the ammoxidation of an alkane.
For example, as a catalyst for use in producing acrylonitrile by ammoxidation of propane, there is known an oxide catalyst containing molybdenum, vanadium, tellurium and niobium. Such a catalyst and a method for producing the catalyst are disclosed in each of Japanese Patent Publication No. 2608768 (published in 1997)(corresponding to U.S. Pat. No. 5,049,692), Unexamined Japanese Patent Application Laid-Open Specification No. 5-208136 (corresponding to U.S. Pat. No. 5,281,745) and Unexamined Japanese Patent Application Laid-Open Specification No. 6-285372 (corresponding to U.S. Pat. No. 5,422,328). Further, Unexamined Japanese Patent Application Laid-Open Specification No. 9-157241 (corresponding to European Patent No. 767,164) discloses an oxide catalyst containing molybdenum, vanadium, antimony and niobium as a catalyst for use in producing acrylonitrile by ammoxidation of propane. These prior art documents have a description concerning ammoxidation reaction conditions employed in the evaluation of the performance of such catalyst. However, with respect to the manner of elevating the temperature of the catalyst before starting the ammoxidation reaction of propane, no description is found in these prior art documents.
Unexamined Japanese Patent Application Laid-Open Specification No. 10-57813 discloses a process for producing acrylic acid by oxidation of propane, using a catalyst containing molybdenum, vanadium and at least one element selected from tellurium and antimony. In this process, the catalyst is subjected to heat treatment virtually in the absence of molecular oxygen, and then subjected to further heat treatment under a stream of air. However, in this prior art document, there is no description as to whether or not such a manner of heat treatment in that process is effective for the ammoxidation reaction of propane or isobutane.
Unexamined Japanese Patent Application Laid-Open Specification No. 8-225506 (corresponding to U.S. Pat. No. 5,534,650) discloses a method for performing an ammoxidation reaction of an alkane in which gaseous ammonia is fed into a reactor from a plurality of ammonia inlets provided therein. However, with respect to the manner of elevating the temperature of a catalyst before starting the ammoxidation reaction of an alkane, no description is found in this prior art document.
Japanese Patent Publication No. 2599677 (published in 1997)(corresponding to U.S. Pat. No. 5,332,855) discloses an ammoxidation method for a saturated hydrocarbon, using a catalyst which contains vanadium, antimony and at least one element selected from iron, gallium and indium; Examined Japanese Patent Application Publication No. 6-92355 (corresponding to U.S. Pat. No. 5,334,743) discloses an ammoxidation method for a saturated hydrocarbon, using a catalyst which contains molybdenum, vanadium and at least one element selected from manganese, zinc, cobalt, copper, lithium, sodium, potassium and silver; and Japanese Patent Publication No. 2506602 (published in 1996)(corresponding to U.S. Pat. No. 5,336,804) discloses an ammoxidation method for a saturated hydrocarbon, using a catalyst which contains vanadium, antimony and bismuth. In these prior art documents, there is a description about an evaluation method for the ammoxidation reaction, in which the temperature of a catalyst contained in a reactor is first elevated to 150.degree. C. while purging the air in the reactor with helium, and then feedstock gases and diluent gases, i.e., propane, ammonia, oxygen, steam and helium, are fed while further elevating the temperature of the catalyst to a predetermined temperature, for example, 300.degree. C., whereupon the catalyst temperature is maintained at the predetermined temperature for 30 minutes, followed by analysis of a gaseous mixture withdrawn from the outlet of the reactor. The present inventors conducted experiments in which the catalysts described in these prior art documents were produced and ammoxidation reactions were conducted using the produced catalysts individually to make various evaluations. As a result, it was found that the performance of the catalyst is almost not affected by the type of gas which is supplied into a reactor containing the catalyst during the elevation of the temperature of the catalyst before starting the ammoxidation reaction. More specifically, the catalyst exhibits the same performance irrespective of whether the gas used during the temperature elevation is air alone or air plus combustible gas selected from feedstock gases.
As a method for the ammoxidation of an olefin by using a catalyst which contains antimony and uranium, British Patent No. 1,304,665 discloses a method in which a catalyst bed comprising a regeneration zone and a reaction zone located adjacent to each other is provided, and molecular oxygen necessary for the ammoxidation is flowed through the regeneration zone of the catalyst bed, together with ammonia in an amount of 3% or more of the whole amount of ammonia to be fed. In this method, the feeding of ammonia to the regeneration zone is intended to suppress a loss of propylene in the regeneration zone. In this prior art document, there is no description as to the manner of elevating the temperature of the catalyst bed before the start of the ammoxidation reaction of propylene.
U.S. Pat. No. 4,814,478 discloses a method in which the ammoxidation reaction of a saturated hydrocarbon is performed using a mixed catalyst of a catalyst comprised mainly of vanadium and antimony and a catalyst comprised mainly of molybdenum, bismuth and iron. In this prior art document, as a specific example of the reaction method, a method using a fixed-bed reactor is described, wherein gaseous raw materials are preheated by means of a "preheat leg", and the resultant preheated gaseous raw materials are then introduced into a catalyst bed. However, this prior art document has no description as to the manner of elevating the temperature of the catalyst bed before starting the ammoxidation reaction.
U.S. Pat. No. 3,833,638 discloses a catalyst for use in the ammoxidation reaction of a saturated hydrocarbon. The catalyst contains molybdenum, cerium and at least one element selected from the group consisting of bismuth and tellurium. In this prior art document, it is described that gaseous raw materials can be fed to the reactor either before or after the internal temperature of the reactor reaches a desired reaction temperature. In this connection, it is noted that the catalyst of this prior art document is of a type such that the regeneration of the catalyst can be performed through contact with air at high temperatures. Therefore, with respect to the technique of this prior art document, it is preferred that the elevation of the internal temperature of the reactor toward a desired reaction temperature is performed while supplying air into the reactor, and the feeding of gaseous raw materials is started after the internal temperature of the reactor has reached the desired reaction temperature.
As described hereinabove, various proposals have been made to provide an improved process for producing an unsaturated nitrile by ammoxidation of an alkane (such as propane and isobutane) in the presence of a catalyst. However, such proposals have been disadvantageous in that the catalytic activity of the specific catalyst (comprising a carrier having supported thereon an oxide catalyst comprised of a compound oxide of molybdenum, vanadium, niobium and at least one element selected from the group consisting of tellurium and antimony) cannot be maintained at a high level when the ammoxidation reaction is performed. Therefore, it has been desired to develop an improved process for producing an unsaturated nitrile from an alkane by ammoxidation, which can maintain the catalytic activity of the catalyst at a high level even after the temperature of the catalyst is elevated to the ammoxidation reaction temperature.